1. Field of the Invention
The present invention relates generally to a method and apparatus for allocating resources of a physical channel in a communication system using Orthogonal Frequency Division Multiplexing (OFDM), and in particular, to a method and apparatus for allocating resources of a downlink control channel.
2. Description of the Related Art
Recently, in mobile communication systems, intensive research has been conducted on Orthogonal Frequency Division Multiplexing (OFDM) as a scheme useful for high-speed data transmission on wire/wireless channels. OFDM, a scheme for transmitting data using multiple carriers, is a kind of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbols, and modulates each of the parallel symbols with multiple orthogonal frequency tones, or multiple orthogonal subcarrier channels before transmission of the parallel symbols.
The MCM-based system was first applied to military high-frequency radios in the late 1950s, and OFDM, which overlaps multiple orthogonal subcarriers, has been in development since the 1970s. However, application of OFDM to actual systems was limited due to the difficulties in realizing orthogonal modulation between multiple carriers. However, Weinstein et al. showed in 1971 that OFDM-based modulation/demodulation can be efficiently processed using Discrete Fourier Transform (DFT), and remarkable technical developments in OFDM have been made over time. Additionally, as OFDM uses a guard interval, and a scheme of inserting a Cyclic Prefix (CP) into the guard interval is known, the OFDM system has noticeably reduced the negative influence for the system's multipath and delay spread.
Owing to such technical developments, OFDM technology is widely applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN) and Wireless Asynchronous Transfer Mode (WATM), i.e., OFDM, which was not widely used, due to hardware complexity, can now be realized with the recent development of various digital signal processing technologies, including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).
OFDM, although similar to the conventional Frequency Division Multiplexing (FDM), is characterized in that OFDM can obtain the optimal transmission efficiency during high-speed data transmission by keeping the orthogonality between multiple tones during the transmission. Additionally, OFDM, having a high frequency efficiency and robustness against multi-path fading, can obtain an optimal transmission efficiency during high-speed data transmissions. OFDM provides several other advantages. Since OFDM overlaps frequency spectra, OFDM has a high frequency efficiency, is robust against frequency-selective fading, and impulse noises, can reduce an Inter-Symbol Interference (ISI) influence using the guard interval, and enables simple designs of hardware equalizers. Therefore, there is an increasing tendency for OFDM to be actively used for communication system configurations.
In wireless communications, high-speed, high-quality, data services are hindered mainly due to channel environments. The channel environments are subject to frequent change, not only due to Additive White Gaussian Noise (AWGN), but also due to a received signal's power variation caused by a fading phenomenon, shadowing, a Doppler effect based on movement and frequent velocity change of a terminal, and interference to/from other users and multipath signals. Therefore, in order to support high-speed, high-quality data services in wireless communications, there is a need to effectively address impeding factors.
In OFDM, a modulation signal is transmitted via allocated two-dimensional time-frequency resources. Resources on the time domain are classified into different OFDM symbols, and the OFDM symbols are orthogonal to each other. Resources on the frequency domain are classified into different tones, and the tones are also orthogonal to each other, i.e., in OFDM, it is possible to indicate a unit resource by appointing a particular OFDM symbol on the time domain and a particular tone on the frequency domain, and the unit resource is called a Resource Element (RE). As different REs are orthogonal to each other, even though they experience a selective channel, signals transmitted on different REs can be received without mutual interference.
A physical channel is a channel of a physical layer that transmits a modulation symbol obtained by modulating at least one coded bit stream. An Orthogonal Frequency Division Multiple Access (OFDMA) system generates and transmits multiple physical channels according to the use of a transmission information stream or the receiver. A transmitter and a receiver should previously agree on the rule for determining for which REs the transmitter and receiver will arrange one physical channel during transmission of the REs, and this rule is called ‘mapping’.
Mapping rules may vary according to the application feature of the particular physical channel. When the transmitter maps a physical channel using a scheduler to increase the system's transmission efficiency in the state where the transmitter perceives a state of a received channel, it is preferable to arrange one physical channel on a set of REs having similar channel states, and when the transmitter maps a physical channel, while aiming to decrease a reception error rate in the state where the transmitter fails to perceive a state of the received channel, it is preferable to arrange one physical channel on a set of REs expected to have very different channel states. The former scheme is mainly suitable for cases where the transmitter transmits data for one user who is insusceptible to a time delay, and the latter scheme is mainly suitable for cases where the transmitter transmits data or control information for one user who is susceptible to the time delay, or transmits data or control information to a plurality of users. The latter scheme uses resources having different channel states in order to obtain diversity gain, and within one OFDM symbol, frequency diversity gain can be obtained by mapping a physical channel to subcarriers that are spaced as far apart as possible on the frequency domain.
Recently, in the 3rd Generation Partnership Project (3GPP), a standardization work for a radio link between a Node B (also known as a Base Station (BS)) and a User Equipment (UE; also known as a Mobile Station (MS)) has been conducted in the name of a Long Term Evolution (LTE) system. The LTE system is most characterized by adopting OFDMA and Single Carrier Frequency Domain Multiple Access (SC-FDMA) as multiplexing schemes of the downlink and the uplink, respectively. The present invention proposes a method for mapping control channels of the LTE downlink to REs.
FIG. 1 illustrates a subframe structure in a general LTE system.
One Resource Block (RB) is composed of 12 tones in the frequency domain and 14 OFDM symbols in the time domain. RB #1 111 represents the first RB, and FIG. 1 shows a bandwidth composed of a total of K RBs from RB #1 111 to RB #K 113. In the time domain, 14 OFDM symbols constitute one subframe 117, and become a basic unit of resource allocation in the time domain. One subframe 117 has a length of, for example, 1 ms, and is composed of two slots 115.
A Reference Signal (RS), which is agreed upon with a Node B so that a UE can perform channel estimation, is transmitted, and RS0 100, RS1 101, RS2 102 and RS3 103 are transmitted from antenna ports #1, #2, #3 and #4, respectively. If only one transmit antenna port is used, RS1 101 is not used for transmission, and RS2 102 and RS3 103 are used for transmission of data or control signal symbols. If two transmit antenna ports are defined, RS2 102 and RS3 103 are used for transmission of data or control signal symbols.
On the frequency domain, though the absolute positions of REs where RSs are arranged are set differently for each cell, a relative interval between RSs is kept constant, i.e., RSs for the same antenna port maintain a 6-RE interval, and a 3-RE interval is maintained between RS0 100 and RS1 101, and between RS2 102 and RS3 103. The absolute positions of RSs are set differently for each cell in order to avoid inter-cell collision of RSs.
Meanwhile, a control channel is disposed in the forefront of one subframe on the time domain. In FIG. 1, reference numeral 119 shows a region where a control channel can be disposed. A control channel can be transmitted over L leading OFDM symbols of a subframe, where L=1, 2 and 3. When the control channel can be sufficiently transmitted with one OFDM symbol as an amount of data to be transmitted is small, only 1 leading OFDM symbol is used for control channel transmission (L=1), and the remaining 13 OFDM symbols are used for data channel transmission. When the control channel uses 2 OFDM symbols, only 2 leading OFDM symbols are used for control channel transmission (L=2), and the remaining 12 OFDM symbols are used for data channel transmission. When the control channel uses all of 3 OFDM symbols as the amount of data to be transmitted is large, 3 leading OFDM symbols are used for control channel transmission (L=3), and the remaining 11 OFDM symbols are used for data channel transmission.
The reason for disposing the control channel in the forefront of a subframe is to allow a UE to determine whether the UE will perform a data channel reception operation by first receiving the control channel and perceiving the existence of a data channel transmitted to the UE itself. Therefore, if there is no data channel transmitted to the UE itself, the UE has no need to perform data channel reception, making it possible to save the power consumed in the data channel reception operation.
The downlink control channel, defined by the LTE system, includes a Physical Channel Format Indication CHannel (PCFICH), a Physical H-ARQ (Hybrid-Automatic Repeat reQuest) Indicator Channel (PHICH), and a Packet Dedicated Control CHannel (PDCCH). A PCFICH is a physical channel for transmitting Control Channel Format Indicator (CCFI) information. CCFI is 2-bit information for indicating a region L where the control channel can be disposed. Because the UE cannot receive the control channel until the first receives CCFI, PCFICH is a channel that all UEs must first receive in the subframe, except when downlink resources are fixedly (persistently) allocated. Further, since the UE cannot know the region L before the UE receives PCFICH, PCFICH should be transmitted in the first OFDM symbol. A PHICH is a physical channel for transmitting a downlink ACK/NACK signal. A UE receiving a PHICH is a UE that is performing data transmission on the uplink. Therefore, the number of PHICHs is proportional to the number of UEs that are now performing data transmission on the uplink. The PHICH is transmitted in the first OFDM symbol (LPHICH=1), or transmitted over three OFDM symbols (LPHICH=3). LPHICH is a parameter defined for every cell, and for a large-sized cell, since there is difficulty in transmitting the PHICH only with one OFDM symbol, the parameter LPHICH is introduced to adjust it. PDCCH is a physical channel for transmitting data channel allocation information or power control information.
For the PDCCH, a channel coding rate can be differently set according to a channel state of a UE that receives the PDCCH. Since the PDCCH fixedly uses Quadrature Phase Shift Keying (QPSK) as a modulation scheme, the amount of resources used by one PDCCH should be changed in order to change the channel coding rate. A high channel coding rate is applied to a UE having a good channel state to reduce the amount of resources used. However, a low channel coding rate is applied to a UE having a poor channel state even though the amount of resources used is increased, thus enabling normal reception. The amount of resources consumed by individual PDCCHs is determined in units of Control Channel Elements (CCEs). For a UE having a good channel state, the PDCCH is composed of only one CCE, and for a UE having a poor channel state, the PDCCH is generated using a maximum of 8 CCEs. The number of CCEs used for generating one PDCCH is one of 1, 2, 4 and 8. One CCE is composed of a set of NCCE mini-CCEs. A mini-CCE is a set of 4 consecutive REs except for the RE used for an RS on the frequency domain. For NCCE=9, the number of REs used for generating one PDCCH is one of 36, 72, 144 and 288.
A mini-CCE is a basic unit of resources constituting a PCFICH and a PHICH. The PCFICH and the PHICH use a predetermined amount of resources, and in order to ease the application of multiplexing with PDCCH and transmission diversity, the amount of resources is determined as a set of mini-CCEs. One PCFICH is generated using NPCFICH mini-CCEs, and one PHICH is generated using NPHICH mini-CCEs. For NPCFICH=4 and NPHICH=3; the PCFICH uses 16 REs and the PHICH uses 12 REs.
In order to multiplex several ACK/NACK signals, the PHICH employs a Code Division Multiplexing (CDM) technique. Four PHICHs are CDM-multiplexed to one mini-CCE, and are repeatedly transmitted so that the PHICHs are spaced as far apart as by NPHICH on the frequency domain in order to obtain frequency diversity gain. Therefore, with a use of NPHICH mini-CCEs, 4 or less PHICHs can be generated. In order to generate more than 4 PHICHs, another NPHICH mini-CCEs should be used. If the required number of PHICHs is M, ceil(M/4)×NPHICH mini-CCEs, i.e., 4×ceil(M/4)×NPHICH REs, are used. Here, ceil(x) is a ceiling function used for calculating the minimum integer greater than or equal to x.
In the mobile communication system using OFDM, a description of which has been made with reference to the LTE system, the conventional resource allocation scheme for transmitting a downlink control channel is as follows: When allocation of an RE set for transmission of a control channel is completed in the entire frequency band of the first OFDM symbol period, allocation of an RE set for transmission of a control channel is performed in the entire frequency band of the second OFDM symbol period. In this manner, in the conventional resource allocation scheme, resource allocation for an RE set is performed in a frequency-first manner in each OFDM symbol period used for transmission of a control channel.